1. Field of the Invention
The present invention relates to power amplifier circuits and radio frequency transmitter systems, and particularly to a method for system level oriented load-pull-based envelope tracking power amplifiers.
2. Description of the Related Art
The evolution of wireless communication systems has always been driven by the need to enhance the users' connectivity by accommodating, within the limited radio frequency (RF) spectrum, the largest number of users with increasingly high data throughputs. This was accomplished by combining compact modulation schemes with advanced access technologies, such as orthogonal frequency division and code division-based multiplexing techniques (OFDM and CDMA, respectively). The resulting spectral efficient techniques translate into time domain signals that have strong amplitude fluctuations and high peak-to-average power ratio (PAPR).
The amplitude modulation of these signals makes them highly sensitive to analog RF front-end nonlinear distortions, whereas their high PAPR makes efficient amplification challenging. Thus, the design of modern base station power amplifiers calls for a trade-off between efficiency and linearity. Since linearity is a must in order to meet the spectral emission regulations, the linearity-efficiency trade-off is often biased toward linearity, and the design constraint is formulated as meeting the linearity requirements with the highest possible efficiency.
Several approaches have been considered for power efficient linear amplification in base station radio systems. The trend is to use power-efficient but mildly nonlinear power amplifiers in conjunction with system level linearization techniques (namely, digital predistortion). The main objective is to increase the efficiency of the power amplifier in the back-off region where the signal probability is high in order to enhance the overall average power efficiency when high PAPR signals are transmitted. The most popular solutions are based on dynamic modulation concepts or switching mode amplifiers. These can be categorized in two classes: dynamic load modulation and dynamic bias modulation. Doherty power amplifiers represent the dynamic load modulation approach, where a carrier and a peaking amplifier modulate their mutual load impedances to achieve high efficiency in the back-off region. Doherty power amplifiers are widely used in base station radio systems. However, their adoption for future systems presents several challenges, mainly related to maintaining the load modulation mechanism for broadband and multi-band applications. Envelope tracking (ET) power amplifiers represent the most promising alternative for dynamic bias modulation power amplifiers, which also includes envelope elimination and restoration technique. In ET power amplifiers, the drain supply voltage of the power amplifier is varied as a function of the input signal's envelope in order to reduce the DC power consumption of the amplifier and consequently improve its power efficiency. The advantage of such polar structure is its suitability for broadband and multi-band applications when compared to the Doherty architecture.
Traditionally, the design of any RF power amplifier starts with the load-pull characterization. The results of this step are used to select the appropriate load and source reflection coefficients to be presented at output and input of the transistor, respectively. What is needed is a design approach customized to envelope tracking power amplifiers.
Thus, a method for system level oriented load-pull-based envelope tracking power amplifiers solving the aforementioned problems is desired.